Cold break is additional protein and polyphenol material that precipitates out of wort as it cools rapidly after the boil — a second, distinct clarification event from hot break, triggered specifically by the fast temperature drop a wort chiller produces rather than by boiling itself.
Why chilling speed genuinely affects how much you get
A faster chill produces a stronger, more complete cold break than a slow one — this is one of the real, practical reasons rapid chilling matters beyond simply saving time and reducing contamination exposure: a wort left to cool slowly on its own doesn't drop through the temperature range where cold break forms quickly enough to precipitate as thoroughly, leaving more haze-forming material behind in the finished beer.
Some brewers separate it out, others let it ride into the fermenter
Whether to leave cold break material in the kettle (whirlpooling it to the centre and racking clear wort off the top) or let it transfer into the fermenter along with the wort is a genuine, debated choice among homebrewers — some argue leaving it behind improves clarity and reduces certain off-flavor risks, others argue the yeast handles it fine and the extra transfer step isn't worth the trouble for a homebrew-scale batch.